Allow source plugins to access previous sources

Description

Source plugin implementations can now specify that they need access to previously staged sources by specifying BST_REQUIRES_PREVIOUS_SOURCES_TRACK and/or BST_REQUIRES_PREVIOUS_SOURCES_FETCH, corresponding to access at track and fetch times respectively.

Changes proposed in this merge request:

• Allow source plugins to access previous sources
• Add a pip source plugin

This merge request, when approved, will close issue/bug: #381 (closed)

Replaces !505 (closed). For relevant discussion, see this discussion: !505 (comment 83780747)

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